1. Field of the Invention
The present invention relates to a flow meter, and more particularly to an air flow measuring instrument that comprises an intake system of an automobile engine and is suitable for detecting and controlling an air-intake amount.
2. Background Art
An instrument that measures a flow rate using the amount of heat discharge of a heating resistor obtained by carrying out heat control of the heating resistor, and an instrument that measures a flow rate by carrying out heat control of a heating resistor and measuring temperature changes in a temperature sensing resistor disposed adjacent to the heating resistor and the like are known as air flow measuring instruments that measure an air flow rate. The air flow measuring instrument is mounted in one part of an air intake duct of a vehicle, and fulfills the role of measuring the intake air flow rate. Normally, an air filter is provided in the air intake duct, and dust included in air that flows into the air intake duct is removed with the air filter. However, depending on the size of the dust, some dust may pass through the air filter, or dust may enter inside the air intake duct due to a mounting fault after replacing the air filter. Dust that enters into the air intake duct is accelerated up to a velocity of several tens of m/s together with a fluid when the amount by which the accelerator is depressed increases, and in some cases the dust may reach the inside of an auxiliary passage of the air flow measuring instrument. An air flow measuring element that is disposed inside the auxiliary passage has an extremely thin portion, and the air flow measuring element may be broken by the dust colliding therewith. Further, when dust that enters into the air intake duct adheres to the air flow measuring element of the air flow measuring instrument, there is the possibility that the heat discharge properties of the air flow measuring element will change and cause a change in the output properties.
A thermal air flow measuring instrument is known in which, as a structure for protecting an air flow measuring element from dust and the like that enters the inside of an air intake duct and preventing deterioration over time due to contamination, a heat sensing resistor is arranged at an angle of from 20 to 60 degrees with respect to the flow direction of fluid so that the air flow detection portion is on the undersurface side (see Patent Document 1). Further, an air flow measuring instrument is known in which, in order that an anti-sensing portion faces an upstream direction, an air flow measuring element is mounted so as to be inclined by an angle α with respect to the axial direction of the flow path (see Patent Document 2).    [Patent Document 1] JP Utility Model Publication (Kokoku) No. 6-020974 (1994)    [Patent Document 2] JP Patent Publication (Kokai) No. 2003-262144